iio: adc: ad7944: simplify adi,spi-mode property parsing

iio: adc: ad7944: simplify adi,spi-mode property parsing

This simplifies the adi,spi-mode property parsing by using
device_property_match_property_string() instead of two separate
functions. Also, the error return value is now more informative
in cases where there was a problem parsing the property.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240325-ad7944-cleanups-v3-1-3a19120cdd06@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>